Needle coke, used in the aggregates of graphite electrodes for electric steelmaking, is commonly produced using petroleum-based heavy oil or coal tar as the starting material. In graphite electrode production steps, coke grains and a binder pitch are first mixed in a prescribed proportion and hot-kneaded, and then extrusion-molded to form a green electrode. The green electrode is then fired and graphitized, and machined to obtain a graphite electrode product.
Graphite electrodes are used under harsh conditions, such as in high-temperature atmospheres, and are therefore desired to have low thermal expansion coefficients (CTE). That is, a smaller thermal expansion coefficient reduces electrode wear during electric steelmaking, and can reduce costs in electric steelmaking.
Methods for controlling the thermal expansion coefficient during production of needle coke are therefore being investigated, and various methods have been proposed. For example, Patent document 1 discloses a process in which an oligomer, which is QI-removed pitch obtained by essential removal of the quino line insolubles from a coal tar-based starting material, and having a modified polymerization degree, is added and delayed coking is performed directly for coking. Also, Patent document 2 discloses a process in which a stock oil is prepared by mixing coal tar-based heavy oil and petroleum-based heavy oil in a proportion so that the nitrogen content is no greater than 1.0 wt % and the sulfur content is no greater than 1.4 wt %, and the stock oil is loaded into a delayed coker to produce raw coke, after which the obtained raw coke is calcined in a temperature range of 700-900° C., temporarily cooled, and then re-calcined in a temperature range of 1200-1600° C. Furthermore, Patent document 3 discloses a process in which, during production of coal tar by rapid thermal decomposition of coal, the thermal decomposition temperature in the reactor is kept at above 750° C., and the residence time of the thermal decomposition product in the reactor is limited to no longer than 5 seconds to obtain a liquid product, the liquid product or the pitch contained therein being carbonized. In Patent document 4, there is disclosed a process in which petroleum-based heavy oil alone or a mixture of the petroleum-based heavy oil with coal tar-based heavy oil from which the quinoline-insolubles have been previously removed, is used as stock oil for delayed coking to produce needle coke, for which process the petroleum-based heavy oil used is pre-modified so that the content of particles such as ash is in the range of 0.05-1 wt %.